In digital radiography, computed radiography (CR) is a medical imaging technique that captures the information using storage phosphor screens. The information is stored as a latent image for later read-out by a laser scanning system. The latent image signal remains stable for a certain period of time ranging from minutes to days. Using a red or near-infrared light to stimulate the phosphor screen which will then release its stored energy in the form of visible light, one can observe the phenomenon known as photostimulable luminescence. The intensity of the stimulated luminescence is proportional to the number of x-ray photons absorbed by the storage phosphor. In practice, the read-out process is accomplished using a laser scanner in order to provide high resolution data and also not to flood the screen with stimulating light. The stimulated light emitted from each point on the screen is collected and converted into a digital signal. The screen is erased by flooding the entire screen with high illuminance light. This allows the screen to be reused. Such a CR imaging system is illustrated in FIG. 1. As shown, an x-ray source 10 projects an x-ray beam 12 through a body part 14 of person 16 to produce a latent x-ray image stored in storage phosphor 18. Collimation blades 20 limit the size of the x-ray beam. The storage phosphor 18 is scanned by a laser beam 22 from laser 24 deflected by deflector 26 driven by rotor 28 as storage phosphor 18 is moved in direction 30. Light emitted by storage phosphor 18 is filtered by filter 32 and detected by photodetector 34. The signal produced by photo detector 34 is amplified by amplifier 36, digitized by analog to digital converter (A/D) 38 and stored in memory 40.
In their practice, radiologists may use x-ray opaque materials to limit the radiation scattering, and shield the subjects from unnecessary exposure to x-rays. Such x-ray opaque materials constitute the "collimation". As illustrated in FIG. 1, the collimation blades 20 are placed in between the x-ray source 10 and the subject 16, and are typically closer to the source 10. As a result, those body parts of the patient 16 which are not important to diagnosis, but which may be vulnerable, are protected. Furthermore, the use of collimation can also limit the radiation scattering from unintended regions from fogging the screen.
In general, the resulting digital images need to be enhanced through code-value remapping in order to provide maximum visual contrast in the region of interest prior to display or printing. Such a process is referred to as tone scaling. The dynamic range of the CR systems (over 10,000:1) provides significant advantage over conventional film in terms of exposure latitude so that CR imaging is very tolerable to improper selection of exposure conditions. However, to optimally render such data on desired printing or display devices, it is necessary to develop a tone scale remapping function. To this end, it is desirable to exclude the shadow regions cast by the collimation from the calculation of the histogram statistics because these regions provide no useful information but distort the intensity histogram. Moreover, since the shadow regions are usually with the highest brightness levels (corresponding to minimum density), the flare can be reduced by setting the shadow regions to a comfortable brightness level or reducing their average brightness level. Such a process is referred to as masking.
Radiation field recognition, preferably done automatically, is the key to masking and is also important to tone scaling. However, it is a very challenging task. In CR, we have significant difficulties to overcome: (1) the boundaries between the region of interest and the shadow regions are usually fuzzy due to the radiation scattering, (2) the region of interest often has significant modulation including prominent edges, (3) the shadow regions may have some comparable modulation and therefore are not uniform due to the radiation scattering from the region of interest, (4) boundaries may be invisible near very dense body parts due to the lack of x-ray penetration, or the frequent underexposure in order to minimize the x-ray dosage.
Due to the practical use and the technical difficulties in collimation recognition, there have been considerable efforts on this subject in the past. Thus, U.S. Pat. No. 4,952,807, inventor Adachi, issued August 1990, assumes that the collimation does not touch the object of the image and the search of collimation boundary is by scanning inwards from image borders to detect the object portion surrounded by the background region. The assumption is not always valid, in particular in shoulder or skull examinations. U.S. Pat. No. 4,804,842, inventor Nakajima, issued February 1990, excludes the shadow region by finding the first valley and thus removing the lower part of the histogram. The basic assumption of this patent technique is that the code values of the those pixels inside the radiation field should be higher than those of the pixels in the collimation shadow regions, which is not always valid. These two patents only address the tone scale problem which does not demand an explicit detection of the collimation boundaries. U.S. Pat. No. 4,970,393, inventor Funahashi, issued November 1990, thresholds 1st derivatives along predetermined scan lines. Japan Patent 2,071,247, inventor Takeo, issued March 1990, and U.S. Pat. No. 5,081,580, inventor Takeo, issued January 1992, assumes strong signal-to-shadow boundaries in terms of the amplitude of the 1D differentiation along the radial lines from the center of the image. U.S. Pat. No. 4,995,093, inventors Funahashi et al., issued February 1991, applies Hough Transform to edge pixels with significant gradient magnitude. U.S. Pat. No. 4,977,504, inventor Funashi, issued December 1990, and the U.S. Pat. 5,268,967, inventors Jang et al., issued December 1993, describe a distinctive approach which classifies non-overlapping small tiles. U.S. Pat. No. 4,952,805, inventor Tanaka, issued August 1990, tests a series of possible hypotheses with respect to the characteristics of the inside and the outside histograms to determine the presence or absence of a radiation field. Japan Patent 7,181,609, inventor Takeo et al. issued July 1995, is unique in that it constructs a decision network consisting several parallel recognition processes, including a first means for detecting rectangular boundaries using boundary contrast, a second means for close-contour tracing to explicitly deal with arbitrarily curved shape irradiation field, a third means for irradiation field determination using imaging information (collimation shape and position, exposure, etc.) and exam type information, and a fourth means using the projections. The final radiation is determined through a coordinating procedure.
Other related patents include U.S. Pat. No. 4,829,181, inventor Shimura, issued May 1989 (using 1D linear edge detection and Hough transform), U.S. Pat. No. 4,851,678, inventors Adachi et al., issued July 1989, (using differentiation processing and also assuming higher code values in the radiation field), U.S. Pat. No. 4,914,295, inventors Ishida et al., issued April 1990, (background detection), U.S. Pat. No. 4,962,539, inventors Shimura et al., issued July 1991 (recognizing the layout pattern in the radiation images by binary pattern matching), U.S. Pat. No. 5,032,733, inventors Funahashi et al., issued July 1991 (detecting unexposed regions in multiple exposed images by locating low variation and low signal level regions).